Escalators vs. Elevators

What's the greenest way to get to the second floor?

How much energy do escalators use? They keep those things running all day long! Should I take the elevator instead?

When it comes to energy use, all escalators are not created equal. The bigger an escalator is—the higher it rises and the wider its steps—the more juice it needs to go trudging along its endless, circular path. The amount of traffic it gets also makes a difference.

As with elevators, escalators can vary widely when it comes to energy consumption. According to a representative of Power Efficiency Corporation, a company that designs energy-saving devices for escalators, your average unit in a shopping mall—which has a 7.5 horsepower motor, rises 15 feet above the ground, and is kept running 14 hours a day, six days a week—might use about 7,500 kilowatt-hours of electricity in a year. A bigger escalator that runs all day and all night in a convention center or hotel—say, a 20-foot-high unit with a 20-horsepower motor—would use roughly 31,000 kWh annually. A continuously running escalator of the kind you'd find in airports or subway stations—35 feet high with a 40 horsepower motor—would use around 60,000 kWh annually. (For comparison's sake, the average American home consumes 11,040 kWh in a year.)

So how do moving stairs stack up against elevators? It's a bit of an apples-to-oranges comparison, as the two machines have different jobs. Escalators are great when you're dealing with big throngs of people, since they can carry so many passengers at once. That's why they're so popular in transit stations, where you often have a wave of people needing to exit at the same time. But during periods of light traffic, elevators come out ahead, because they can speedily move small groups of passengers in any direction. Plus, they sit idle—and thus save energy—when not being used. (You can also rig an escalator to stop moving when no one's on it, but that's not a recommended tactic, in part because of liability issues and in part because people who see a stopped escalator tend to assume it's broken.)

Still, escalators seem to be the bigger electricity users—particularly as buildings get taller. Elevator technology consultant Jim Bos modeled two theoretical officebuildings—one with three floors and one with six—each with a peak traffic flow of 35 people per minute. He assumed that the equipment in these scenarios was "reasonably modern" and that the escalators are only kept on 12 hours a day during the workweek. (During the weekend, the escalators are turned off, but the elevators remain on standby.) With the caveat that there are a whole lot of variables in an assessment of this nature, Bos estimates that the shorter building would require either five elevators—which would collectively use 68,000 kilowatt-hours a year—or two pairs of escalators, which would use 75,000 kWh a year. (That includes lighting in the elevator cabs.) To handle the traffic in the six-floor building, you only need one additional elevator—but you have to add three more pairs of escalators so that passengers can enter and exit on each floor. In this scenario, the elevator bank might use 130,000 kWh annually, while the escalators use 187,000 kWh.

Escalators do offer some big opportunities for energy-saving if you can figure out how to cut power consumption during those periods when they chug along without any passengers. Option 1 is to slow down the escalator when it's empty, essentially putting it into sleep mode. These "intermittent" or "variable-speed" escalators are popular in Europe and Asia, but haven't gained much traction in the United States, thanks to a recommended national safety code that forbids escalators from changing speed. (The forthcoming update to the code will lift that injunction for new machines.) The actual savings on a given escalator will depend on how often the unit is idle, but a recent European study estimated that installing variable-speed drives on all the region's escalators could reduce total electricity use by about 28 percent.

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The second option is to install a controller on the escalator motor that improves its efficiency. Escalator motors are designed to move a massive amount of weight at any given time—a "full" escalator would have between 150 and 300 pounds of passenger on each step. A/C motors, like the kind you find in escalators, make the most efficient use of energy when they're moving a full load. But escalators are almost never totally occupied; most of the time, the motor is drawing more power than it actually needs. Motor efficiency controllers try to match a motor's power supply with its exact power needs at any given moment. Manufacturers of these devices claim they can reduce escalators' energy use by 15 to 35 percent. (These controllers can also be installed on elevators.)

In the meantime, there's a simple way to reduce the footprint of your personal escalator habit: Use the stairs on your way up, and ride the escalator on the way down. On an up escalator, each additional passenger makes the motor work a little harder to pull the steps up. But on a down escalator—whose motor is also located at the top of the unit—each additional passenger helps the motor, thanks to the action of gravity. So riding the up escalator costs a little energy, while riding the down escalator saves a little. On the up journey, you'll save the most by skipping the escalator entirely, but you can still save a bit by walking up the moving staircase—you'll finish your journey more quickly that way, and thus the motor won't have to spend as long straining to haul you up.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.